Since the discovery of two-dimensional massless Dirac fermions,
[1]}
extensive studies
have been performed on various materials.
Among them, noticeable phenomena of Dirac electrons
in molecular crystals[2]} have been studied
in the organic conductor
 \(\alpha \) -(BEDT-TTF)\(_2\) I\(_3\)[3]} [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene].
After noting that the density of states (DOS)
vanishes linearly at the Fermi energy,
[4]}
the two-dimensional Dirac cone with a zero-gap state (ZGS)
[5]}
was found using a tight-binding (TB) model, where
transfer energies are estimated from
the extended Hückel method.[6]}
The existence of such Dirac cone was verified by first-principles DFT calculation,
[7]} which has been
used for studying further  \(\alpha \) -(BEDT-TTF)\(_2\) I\(_3\)  under hydrostatic
pressure.[8]}
